1. Field of the Invention
This invention relates to an attachment method and attachment device for battery cells installed in locations subjected to centrifugal forces.
2. Description of the Related Art
Apparatuses which use battery cells as power supplies are not limited to portable equipment, and various utilization configurations are now being developed. Among these applications are automobiles, space observation equipment, and various types of industrial equipment wherewith battery cells need to be used in environments affected by centrifugal forces. In battery cell applications in apparatuses designed for measuring and monitoring air pressure of an automobile tire while the vehicle is moving, for example, the centrifugal force developed by tire revolution acts on the battery cells. The conditions affecting such battery cells when subjected to centrifugal forces are now described.
The chemical reactions that take place in battery cells are based on ion conduction in liquid electrolytes that are present between a positive polarity material and a negative polarity material. In general, liquid electrolytes are present in a condition wherein they are impregnated in separators placed between the positive polarity material and the negative polarity material, in which condition they contribute to oxidation-reduction reactions between positive and negative electrodes. More specifically, if the active material of positive electrode (positive polarity material) is designated P1, and the active material of negative electrode (negative polarity material) is designated N1, then the reaction that proceeds at the positive pole is represented by chemical equation (1), and the reaction that proceeds at the negative pole is represented by chemical equation (2).
Positive Pole: P1+nexe2x86x92P2xe2x80x83xe2x80x83(1) 
Negative Pole: N1xe2x86x92N2+nexe2x80x83xe2x80x83(2) 
When the reactions expressed by the chemical equations (1) and (2) come into contact via the liquid electrolyte, a DC current can be sent to a circuit connected to the battery cell, whereupon the electricity generating reactions of the battery cell proceed. When such battery cell reactions as these are subjected to centrifugal forces, due to electrolyte flow induced by the centrifugal forces, the electrolyte available for contributing to the electricity generating reactions sometimes decreases, resulting in a decline in battery cell performance. This is now described in the case of a graphite-lithium fluoride battery cell, which is one example of a non-aqueous liquid electrolyte battery cell.
In the graphite-lithium fluoride battery cell, as diagrammed in FIG. 1, a negative electrode 2 formed as a disk made from lithium metal and a positive electrode 3 formed as a disk made from a material of which main component is graphite fluoride are stacked in a battery case 5 made of stainless steel, separated by a separator 4 formed of unwoven polypropylene fabric. The interior of this battery case 5 is filled with a liquid electrolyte wherein lithium borofluoride is dissolved in a liquid mixture of dimethoxy ethane (DME) having a low boiling point and either gamma butyrolactone or propylene carbonate having a high boiling point to bring the volumetric concentration thereof to 1 mol/liter. The opening in the battery case 5 is sealed by a sealing plate 1 made of stainless steel that doubles as a negative terminal, with an intervening gasket 6 formed of a polypropylene resin.
When a centrifugal force acts in the thickness direction on a flat battery cell such as this, an electrolyte that exhibits mobility will move in the direction of the centrifugal force toward one or other of the electrodes, whereupon the volume of electrolyte available for contributing to the electricity generating reactions of the battery cell at the other electrode will decrease. This results in a decline in such battery performance factors as discharge capacity and discharge characteristics, as compared to when the battery cell is used in a normal condition unaffected by centrifugal forces.
An object of the present invention is to provide a battery cell attachment method and attachment device for battery cell applications in environments subjected to centrifugal forces, wherewith battery cell performance does not decline.
What is characteristic of the battery cell attachment method of the present invention is that, when a battery cell wherein positive polarity material and negative polarity material are placed in opposition through a separator or separators in a battery case, and a liquid electrolyte is packed between the positive polarity material and the negative polarity material, is attached in an apparatus installed in a place acted on by centrifugal forces, the battery cell is attached in the apparatus so that one side where the negative polarity material exists faces in a direction in which the centrifugal force is acting on the apparatus. By attaching the battery cell so that the negative polarity material faces in the direction of centrifugal force in this manner, a condition is maintained wherein the liquid electrolyte that flows due to the centrifugal force is present on the opposite side of the negative polarity material that becomes the depolarized side during discharge, and battery cell performance deterioration due to centrifugal force is prevented.
What is further characteristic of the battery cell attachment method of the present invention is that, when a battery cell wherein positive polarity material and negative polarity material are placed in opposition through a separator or separators in a battery case, and a liquid electrolyte is packed between the positive polarity material and the negative polarity material, is attached in an apparatus installed in a place acted on by centrifugal forces, the battery cell is attached in the apparatus so that the negative polarity material side is oriented with respect to the direction in which the centrifugal force acts on the apparatus so that the angle of inclination of the battery cell thickness direction is within a range of 0 to 60 degrees. The liquid electrolyte present at the reaction surface of the negative polarity material decreases the more as the inclination of the thickness direction increases from the attachment angle in a battery cell wherein the negative polarity material is oriented in the direction of the centrifugal force. However, if that angle of inclination is within 60 degrees, so long as a condition wherein excessive centrifugal forces act is avoided, the apparatus can be operated in a condition that is not problematic in practice, without suffering any extreme decline in the discharge capacity.
With the attachment method noted in the foregoing, when a battery cell wherein the battery case is formed as a flat shape is divided into two in the thickness direction, and the vacant capacity left after subtracting the volume occupied by the negative polarity material, the positive polarity material, and the separator from the volume in the battery case, in each of the divided portions, is computed, the vacant capacity of the divided portion on the side where the negative polarity material is located is smaller than that of the divided portion on the other side, and it is possible to attach the battery cell with this side having the smaller vacant capacity oriented in the direction of the centrifugal force. The decline in battery performance due to centrifugal forces acting on the battery cell results when liquid electrolyte is not sufficiently present at the surface of the negative polarity material during the course of electricity generating reactions. Accordingly, when the negative polarity material is on the side of the battery cell having the smaller vacant capacity, and the battery cell is attached so that this side is in the direction of the centrifugal force, a condition is maintained wherein the liquid electrolyte is present at the surface of the negative polarity material, wherefore the decline in battery performance due to centrifugal force is checked.
The battery cell attachment method of the present invention is a method for attaching a battery cell wherein positive polarity material and negative polarity material are placed in opposition through a separator in a battery case, and that battery case is packed with a liquid electrolyte, in an apparatus installed in a place acted on by centrifugal forces, wherein the negative polarity material side of the battery cell is oriented toward the direction in which centrifugal force acts on the apparatus, and the battery cell is loaded in a prescribed position in the apparatus using attachment means that regulate the battery cell loading direction so that the angle of inclination of the battery cell thickness direction relative to the centrifugal force direction is within a prescribed range. Thus the negative polarity material side of the battery cell is regulated so that it is oriented with respect to the direction of centrifugal force within a prescribed angular range, a state is obtained wherein the liquid electrolyte is sufficiently present when centrifugal force acts on the reaction surface of the negative polarity material, and the battery cell can be attached in a condition wherein battery performance does not decline due to centrifugal force.
The attachment means noted above are configured so that the battery cell can be loaded in such manner that the loading direction is regulated by a battery cell attachment structure wherein, of the positive polarity side and negative polarity side having mutually different outer shapes, only the shape on the side of one pole fits.
The battery cell used in the attachment method and attachment device noted in the foregoing is made by placing a positive polarity material formed primarily of either a metal oxide, halide, or sulfide, and a negative polarity material made primarily of either light metal or light metal alloy, so that they oppose each other through a separator made of a substance capable of withstanding temperatures in excess of 150xc2x0 C., into a battery case that functions also as the positive terminal; packing a liquid electrolyte consisting of an organic solvent having a boiling point of 170xc2x0 C. or higher into which is dissolved a solute, in which a lithium salt is used, between the positive polarity material and the negative polarity material; and sealing the opening in the battery case with a sealing plate that also functions as the negative terminal, with an intervening gasket that is resistant to organic solvents and resistant to temperatures in excess of 150xc2x0 C. Thus, by forming the separator, gasket, and liquid electrolyte of materials capable of withstanding high temperatures, vaporization of the liquid electrolyte by high temperature is suppressed, and deterioration in the separator and gasket due to high temperature is prevented. Simultaneous exposure to high temperatures often occurs in environments acted on by centrifugal forces, wherefore battery cells having a heat-resistant structure are effective.